Bug 1689285 - Fix test_visibility.xhtml to not depend on theme values. r=eeejay

It's relying on the other menuitems being tall enough so that they
overflow 5em. When the non-native theme is enabled, this assumption
breaks because that theme doesn't add padding to XUL menuitems, and we
get the failure:

    wrong state bits for 'mi_file1.2.4' !got '0', expected 'offscreen'

Get what we want to test more explicitly by setting the height of the
menuitems appropriately.

Differential Revision: https://phabricator.services.mozilla.com/D103267

accessible/tests/mochitest/states/test_visibility.xhtml

@@ -134,11 +134,11 @@
           <menupopup>
             <menuitem label="SubFile" id="mi_file1.1"/>
             <menu label="SubFile2" id="mi_file1.2">
-              <menupopup style="max-height: 5em;">
-                <menuitem label="SubSubFile" id="mi_file1.2.1"/>
-                <menuitem label="SubSubFile2" id="mi_file1.2.2"/>
-                <menuitem label="SubSubFile3" id="mi_file1.2.3"/>
-                <menuitem label="SubSubFile4" id="mi_file1.2.4"/>
+              <menupopup style="max-height: 3em;">
+                <menuitem style="height: 1em" label="SubSubFile" id="mi_file1.2.1"/>
+                <menuitem style="height: 1em" label="SubSubFile2" id="mi_file1.2.2"/>
+                <menuitem style="height: 1em" label="SubSubFile3" id="mi_file1.2.3"/>
+                <menuitem style="height: 1em" label="SubSubFile4" id="mi_file1.2.4"/>
               </menupopup>
             </menu>
           </menupopup>